Lyme disease is caused by the spirochete Borrelia burgdorferi and is the leading arthropod-borne infection in the United States. In endemic areas, B. burgdorferi infection, if untreated, results in significant morbidity, and in late stages manifets as severe arthritis. A common requirement of all pathogens is the ability to attach to host structures in order to establish an infectious focus. For B. burgdorferi this involves colonization of both the vertebrate host and an arthropod vector, specifically the Ixodes spp. tick. Once the spirochete is deposited into the dermis of a mammal by an infected tick, it binds to components of the extracellular matrix (ECM) where it replicates and subsequently disseminates throughout the host. In this application, we present new data demonstrating a novel mammalian binding target for OspC, specifically thrombospondin (TSP) proteins. Although OspC may bind additional TSP's, we have focused our efforts on its ability to engage TSP5, or cartilage oligomeric matrix protein (COMP), given its abundance in joint tissues and its linkage to arthritis This interaction is particularly interesting given the known dependence of OspC in borrelial pathogenesis and the lack of information pertaining to its function during mammalian infection. Although other activities are known for OspC (e.g., plasminogen binding), there are additional borrelial proteins that can mediate these functions. Thus, the interaction with TSP/COMP represents a novel mammalian target for OspC. Herein, we hypothesize that OspC::COMP interactions are required for the full pathogenic potential of B. burgdorferi and propose to characterize this interaction further with the following Specific Aims: (1) Define the molecular nature of the OspC::COMP interaction. Here we will assess the binding of OspC to COMP specific peptides and recombinant COMP fragments using biochemical and biophysical methods to determine the affinity of this interaction. Furthermore, this data should define the residues in OspC needed for COMP recognition; and (2) Determine the role of COMP in borrelial pathogenesis. We will use in vivo imaging, following murine infection in COMP knockout mice, to assess B. burgdorferi colonization both temporally and spatially and whether the OspC::COMP engagement contributes to arthritis. When completed, the data obtained will provide the metrics necessary to evaluate the importance of the OspC::COMP interaction in borrelial pathogenesis and will provide the foundation needed to further characterize this host-pathogen interaction.